Synthesis and chiroptical properties of optically active, regioregular oligothiophenes

Article abstract of DOI:10.1021/ol016189g

(matrix presented) A series of regioregular oligothiophenes bearing chiral oxazoline residues have been synthesized in high yields by a stepwise synthesis using the Stille cross-coupling reaction. The chiroptical properties of the oligothiophenes up to th

Full text of DOI:10.1021/ol016189g

ORGANIC
LETTERS
2001
Vol. 3, No. 15
2379-2382
Synthesis and Chiroptical Properties of
Optically Active, Regioregular
Oligothiophenes
Shin-ichiro Sakurai, Hidetoshi Goto, and Eiji Yashima*
Department of Molecular Design and Engineering, Graduate School of Engineering,
Nagoya UniVersity, Chikusa-ku, Nagoya 464-8603, Japan
yashima@apchem.nagoya-u.ac.jp
Received May 29, 2001
ABSTRACT
A series of regioregular oligothiophenes bearing chiral oxazoline residues have been synthesized in high yields by a stepwise synthesis using
the Stille cross-coupling reaction. The chiroptical properties of the oligothiophenes up to the octamer were investigated with UV−visible and
CD spectroscopies. The octamer showed split-type Cotton effects in chloroform as a good solvent in the presence of a poor solvent such as
acetonitrile, while the lower molecular weight oligomers exhibited almost no induced CD.
The design and synthesis of chiral π-conjugated polymers
and oligomers with a well-defined structure has attracted
considerable interest in the past decade1 due to their potential
applications not only in optoelectronic devices2 but also in
enantioselective sensors, catalysts, and adsorbents.1a,b,3 The
chiral π-conjugated polymers and oligomers that have been
most intensively studied in this respect are the chiral
regioregular (head-to-tail, HT) polythiophenes (PTs).4 Chiral
PTs are significantly different from other optically active
polymers,5 and they exhibit optical activity in the π-π*
transition region derived from the main chain chirality when
they are aggregated to form a π-stacked supramolecular
aggregate through intermolecular interactions in a poor
solvent, at low temperature, or in a film, whereas they show
no optical activity in this region in a good solvent or at high
temperatures.4 Nevertheless, a complete structural interpreta-
tion on the origin of the optical activity in the aggregated
chiral PTs still remains unclear. Langeveld et al. proposed
(1) Reviews: (a) Pu, L. Acta Polym. 1997, 48, 116. (b) McQuade, D.
T.; Pullen, A. E.; Swager, T. M. Chem. ReV. 2000, 100, 2537. For recent
references, see: (c) Akagi, K.; Piao, G.; Kaneko, S.; Sakamaki, K.;
Shirakawa, H.; Kyotani, M. Science 1998, 282, 1683. (d) Ohkita, M.; Lehn,
J.-M.; Baum, G.; Fenske, D. Chem. Eur. J. 1999, 5, 3471. (e) Prince, R.
B.; Barnes, S. A.; Moore, J. S. J. Am. Chem. Soc. 2000, 122, 2758. (f)
Kilbinger, A. F. M.; Schenning, A. P. H. J.; Goldoni, F.; Feast, W. J.; Meijer,
E. W. J. Am. Chem. Soc. 2000, 122, 1820. (g) Oda, M. Nothofer, H.-G.;
Lieser, G.; Scherf, U.; Meskers, S. C. J.; Neher, D. AdV. Mater. 2000, 12,
362. (h) Nakashima, H.; Fujiki, M.; Koe, J. R.; Motonaga, M. J. Am. Chem.
Soc. 2001, 123, 1963.
(2) For recent reviews, see: (a) Roncali, J. Chem. ReV. 1997, 97, 173.
(b) Hide, F.; D´ıaz-Garcia, M. A.; Schwartz, B. J.; Heeger, A. J. Acc. Chem.
Res. 1997, 30, 460. (c) Swager, T. M. Acc. Chem. Res. 1998, 31, 201. (d)
Martin, R. E.; Diederich, F. Angew. Chem., Int. Ed. 1999, 38, 1350. (e)
Tour, J. M. Acc. Chem. Res. 2000, 33, 791. (f) Kim, D. Y.; Cho, H. N.;
Kim, C. Y. Prog. Polym. Sci. 2000, 25, 1089.
(4) (a) Lemaire, M.; Delabouglise, D.; Garreau, R.; Guy, A.; Roncali, J.
J. Chem. Soc., Chem. Commun. 1988, 658. (b) Bouman, M. M.; Havinga,
E. E.; Janssen, R. A. J.; Meijer, E. W. Mol. Cryst. Liq. Cryst. 1994, 256,
439. (c) Bidan, G.; Guillerez, S.; Sorokin, V. AdV. Mater. 1996, 8, 157. (d)
Langeveld-Voss, B. M. W.; Christiaans, M. P. T.; Janssen, R. A. J.; Meijer,
E. W. Macromolecules 1998, 31, 6702. (e) Fujiki, M.; Nakashima, H.; Koe,
J. R.; Takigawa, H. Polym. Prepr. 1999, 40 (1), 523.
(5) Reviews: (a) Okamoto, Y.; Nakano, T. Chem. ReV. 1994, 94, 349.
(b) Nolte, R. J. M. Chem. Soc. ReV. 1994, 11. (c) Green, M. M.; Peterson,
N. C.; Sato, T.; Teramoto, A.; Cook, R.; Lifson, S. Science 1995, 268,
1860. (d) Gellman, H. S. Acc. Chem. Res. 1998, 31, 173.
(3) Reviews: (a) Goldenberg, L. M.; Bryce, M. R.; Petty, M. C. J. Mater.
Chem. 1999, 9, 1957. (b) Englebienne, P. J. Mater. Chem. 1999, 9, 1043.
(c) Pu, L. Chem. ReV. 1998, 98, 2405.
10.1021/ol016189g CCC: $20.00 © 2001 American Chemical Society
Published on Web 07/03/2001

Scheme 1a
a Reagents and conditions: (i) NBS, DMF, rt, 4 days; (ii) n-BuLi, Me3SiCl, THF, -98 °C, 20 min; (iii) LDA, Me3SnCl, THF, -78 °C,
20 min; (iv) Pd2(dba)3, P(PhO)3, CuO, DMF, 100 °C, 15 h.
the helical packing of predominantly planar chains for the
chiral aggregated PTs rather than a helical cisoidal confor-
mation.6
catalyzed, CuO-modified Stille cross-coupling9 with 2 af-
forded HT-5a in 85% yield together with the head-to-head
(HH) byproduct 6 (6%), which could be easily separated by
silica gel chromatography. In the Stille coupling reaction,
P(PhO)3 was superior to PPh3 as the palladium ligand. When
PPh3 was used as the ligand under the same conditions in
the Stille coupling reaction, the yield (45%) and selectivity
of HT-5a (5a:6 ) 10:1) decreased.10 The trimethylsilyl
protecting group was removed with tetrabutylammonium
fluoride (TBAF) in THF to give the HT-dimer (7a) in 96%
yield. In the same way, the HT-tetramer (7b) and HT-octamer
(7c) were prepared in high yields without difficulty. 7a is a
colorless solid (mp 45 °C), while 7b and 7c are pale yellow
We have previously shown that an optically active HT-
PT, poly[(R)-3-(4-(4-ethyl-2-oxazolin-2-yl)phenyl)thiophene]
(PEOPT), exhibited a unique induced circular dichroism
(ICD) upon complexation with metal ions in a good solvent,
chloroform, for PEOPT, accompanied by negligible changes
in the UV-visible spectra, which indicates that chirality
induction may not be derived from the chiral π-stacked
aggregations of PEOPT but from the main chain chirality
such as a predominantly one-handed helical structure induced
by intermolecular coordination of the oxazoline groups to
the metal ions.7 We also found dramatic changes in the ICDs
of PEOPT depending on the nature of the solvent; the
addition of a poor solvent such as methanol or acetonitrile
to the chloroform solution of PEOPT induced a conforma-
tional change in PEOPT which caused chiral aggregations
with different handedness.8 Here, we report the synthesis and
chiroptical study of a series of optically active oligo-
thiophenes with well-defined structures as the model oligo-
mers of PEOPT. The present model studies will serve to
elucidate the origin of the optical activity in chiral PTs.
The regioregular oligomers (7a-7c) were prepared starting
from optically pure (R)-3-(4-(4-ethyl-2-oxazolin-2-yl)phen-
yl)thiophene monomer (1)7 according to Schemes 1 and 2.
The regioselective bromination at the 2-position of 1 using
N-bromosuccinimide (NBS), followed by transmetalation
with n-BuLi and then quenching with trimethylsilyl chloride
afforded 3 in high yield. Regioselective stannylation at the
5-position of 3 quantitatively gave 4. Subsequent Pd-
Scheme 2a
a Reagents and conditions: (i) TBAF, THF, rt, 20 min; (ii) NBS,
DMF, rt, 3 days; (iii) LDA, Me3SnCl, THF, -78 °C, 20 min; (iv)
Pd2(dba)3, P(PhO)3, CuO, DMF, 100 °C, 6 h.
(6) Langeveld-Voss, B. M. W.; Janssen, R. A. J.; Meijer, E. W. J. Mol.
Struct. 2000, 521, 285.
(7) Yashima, E.; Goto, H.; Okamoto, Y. Macromolecules 1999, 32, 7942.
(8) Goto, H.; Yashima, E.; Okamoto, Y. Chirality 2000, 12, 396.
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Org. Lett., Vol. 3, No. 15, 2001

(mp 112 °C) and orange solids (decomposition at 165 °C),
respectively. All the oligomers were purified by silica gel
chromatography or by preparative, recycle HPLC and
1
characterized and identified using H and 13C NMR spec-
troscopies, elemental analyses, and mass measurements (see
Supporting Information).
The chiroptical properties of these optically active oligo-
mers were investigated using UV-visible and CD spec-
troscopies. The UV-visible spectra of the oligothiophenes
in chloroform exhibit a tendency well correlated with the
number of repeating units (n) of the oligomers (Figure 1).
Figure 2. CD (A) and UV-visible (B) spectral changes of the
octamer (7c) in acetonitrile-chloroform mixtures in a 1.0 cm quartz
cell at rt just after the preparation of the sample solutions. The
concentration of 7c is 0.025 mg/mL.
changed from yellow-orange to purple. These significant
changes in the ICDs were accompanied by a gradual red-
shift in the absorption with a clear isosbestic point (Figure
2B). However, the ICD intensity was reduced to one-half
compared to that of PEOPT and almost twice as much poor
solvent by volume was required for the appearance of the
ICD for the octamer. The dimer and tetramer showed almost
no ICD even in the presence of excess poor solvents (99%,
v/v). Similar ICDs with solvatochromism have been observed
for PEOPT, but the Cotton effect signs were significantly
dependent on the solvents; the addition of methanol and
acetonitrile to the chloroform solution of the PEOPT induced
a conformational change in the PEOPT which caused
opposite chiral aggregations, thus showing mirror image
ICDs.8
However, as for the octamer, the ICD signs were the same
and independent of the solvents. The reason is not clear at
the present. The CD and absorption of the octamer in the
mixtures of chloroform and poor solvents were found to be
highly time dependent, and the ICD intensities increased with
time (Figure 3). These results are in contrast to PEOPT,8
indicating that the intermolecular π-stacked aggregation
formations that strongly depend on the π-conjugation length
and polymers tend to aggregate in a short time for the
chirality induction on the main chain. Similar time-dependent
folding was reported by Moore and co-workers for chiral,
apolar m-phenylene ethynylene oligomers.11
Figure 1. UV-visible spectra of oligothiophenes and HT-PEOPT
in chloroform in a 0.10 cm quartz cell at rt with a concentration of
0.25 mg/mL.
The absorbance maximum (λmax) for the π-π* transition
nonlinearly increases as n increases (λmax: 7a (275), 7b (390),
and 7c (435 nm)). A similar tendency was observed for the
maximum emission wavelength (λem: 7a (447), 7b (510),
and 7c (561)), although HT-PEOPT having an extended
π-conjugation (n ) ca. 20) has red-shifted absorption (λmax
) 452 nm; Figure 1) and emission maximum (λem ) 573
nm). The oligomers exhibit an emission with different colors
from blue (7a) to green (7b) and yellow (7c) both in
chloroform solution and in the solid and are correlated with
the emission maximum.
CD measurements were performed in order to study the
molecular weight dependent, chiral aggregation formation
in solution in the presence of poor solvents or Cu ions. The
oligomers showed no ICD in chloroform in the π-π*
transition region as anticipated for the chiral PTs in random
conformations.4 In sharp contrast, the octamer exhibited an
apparent, split-type ICD at longer wavelengths (350-650
nm) upon the addition of a poor solvent such as methanol
(87.5-97.5%, v/v) or acetonitrile (Figure 2A; 70-75%, v/v)
to the chloroform solution, and the solution spontaneously
In the presence of Cu ions (copper(I) triflate (Cu(OTf))
and Cu(OTf)2), the octamer also exhibited a weak ICD in
the main chain in chloroform accompanied by slight changes
in the UV-visible spectra. The ICD patterns were much
different from those of the PEOPT-Cu complexes in
chloroform (see Supporting Information). These results
suggest that the octamer is not long enough to form a chiral
complex such as the PEOPT-Cu complexes.
(9) McCullough, R. D.; Ewbank, P. C.; Loewe, R. S. J. Am. Chem. Soc.
1997, 119, 633.
(10) Farina, V.; Krishnan, B. J. Am. Chem. Soc. 1991, 113, 9585.
(11) Brunsveld, L.; Prince, R. B.; Meijer, E. W.; Moore, J. S. Org. Lett.
2000, 2, 1525.
Org. Lett., Vol. 3, No. 15, 2001
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pure, achiral oligothiophenes up to a 48-mer13 are known,
the optically active octamer prepared in this study may be
among the longest class of oligomers with optical activity.
Further studies including the synthesis of longer oligomers,
X-ray structure determinations, and scanning tunneling
microscopic analyses14 of the optically active oligomers will
be useful to elucidate the origin of the optical activity in
chiral PTs. This work is now in progress.
Acknowledgment. This work was supported in part by
a Grant-in-Aid for Scientific Research from the Ministry of
Education, Culture, Sports, Science and Technology, Japan.
Supporting Information Available: Experimental pro-
cedures, analytical data of the oligomers, and CD spectra of
7c and PEOPT with Cu ions. This material is available free
of charge via the Internet at http://pubs.acs.org.
Figure 3. Changes in CD (A) and UV-visible (B) spectra of 7c
with time in an acetonitrile-chloroform mixture (72.5%, v/v) in a
1.0 cm quartz cell at 25 °C. The concentration of 7c is 0.025 mg/
mL.
OL016189G
(12) Krischbaum, T.; Briehn, C. A.; Ba¨uerle, P. J. Chem. Soc., Perkin
Trans. 1 2000, 1211.
(13) Nakanishi, H.; Sumi, N.; Aso, Y.; Otsubo, T. J. Org. Chem. 1998,
63, 8632.
(14) (a) Azumi, R.; Go¨tz, G.; Debaerdemaeker, T.; Ba¨uerle, P. Chem.
Eur. J. 2000, 6, 735. (b) Mena-Osteritz, E.; Meyer, A.; Langeveld-Voss,
B. M. W.; Janssen, R. A. J.; Meijer, E. W.; Ba¨uerle, P. Angew. Chem., Int.
Ed. 2000, 39, 2680.
In summary, we have synthesized a series of oligo-
thiophenes with well-defined regiospecificity (HT), chirality,
and molecular weights. Although achiral HT-oligo(3-
substituted thiophene)s up to a 12-mer12 and isomerically
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Org. Lett., Vol. 3, No. 15, 2001